The oldest and still most prevalent method for recognizing or verifying respectively, the identity of a person is face recognition.
In difference to the comparison of fingerprints or signatures or a genetic comparison (“genetic fingerprint”) the visual comparison employed in face recognition can be readily and quickly be performed by the unskilled person without any technical means.
Identity verification by face recognition is always performed by a comparison of two images: (1) a reference image that is either available as a real portrait, e.g., as a photo on an identity card, a search photo or as a different portrait, or as a physical or imaginary image reconstructed from the memory or a description, e.g., a photofit picture, with (2) a comparative image to be compared with the reference image, whereupon the identity is asserted or not depending on the grade of correspondence.
Such kinds of identity verification are usually still performed by persons rather than being executed automatically, as, e.g. in a traffic check or passport control in which cases the reference image is a photo on an identity card and the comparative image is an imaginary image reconstructed by the controlling person from the visual impression of the face of the checked person. Methods of automatic image processing are very laborious due to the huge amount of image information to be processed and also error-prone, if the quality of the used images is insufficient. Features that are not considered as quality features may also play some role. For example, the posture of the head has a huge impact on the significance of the outcome of an automatic comparison of images: according to the art front views qualify much better than other postures as, e.g., the half-side profile of a person's face.
Either identity verification is performed visually be a person or automatically, the quality of the used portraits is a crucial criterion for the significance of an identity verification. For the quality not only the reproduction qualities as the resolution, sharpness, contrast, brightness, staining etc. of the image but also the mode of presentation of the displayed face is of importance. Particularly, for automatic methods portraits of reduced quality represent almost unsolvable problems in successfully comparing images.
Identity verification of persons by comparison of fingerprints, signatures, etc. is to an increasing degree performed with the assistance of semi-automatic or automatic methods. Again, the quality of the used fingerprints or signatures, respectively, is crucial. With respect to signatures the U.S. Pat No. 6,363,162 A, e.g., describes a system and method for verifying the quality of signatures wherein a binary image of the document carrying the signature is generated, the background is eliminated and subsequently the signature is examined and evaluated. However, this method is of no or only very limited use for the application to portraits due to the higher complexity of such images.
In fact, methods and systems for automatically verifying and, if need be, correcting the contrast, color and brightness of images are known in typography and photography. These are, however, always based on an analysis of either the entire image or predetermined image areas. They are not appropriate for the extraction of structures or even identification of individual objects within the image.
A further problem lies in the fact that quality standards for portraits are only vaguely defined and up to now are only verified by visual inspection. The general appearance of a German passport photograph, e.g., is defined by some criteria and examples. However, whether a photograph is acceptable for a passport or not is decided by one or more executives/appointees.
A more precise definition of quality standards is, however, meaningful only, if observation thereof can be verified objectively. An objective inspection method for portraits including verification whether an image actually observes predetermined criteria or not is, however, not available as yet.